Organic electroluminescence panel and method for manufacturing the same

ABSTRACT

On a cathode ( 40 ) of an organic EL element, a stress reducing layer ( 42 ) formed by a material which is the same as that used for an organic layer of the organic EL element is formed. A moisture block layer ( 44 ) formed by a material which is the same as that used for the cathode ( 40 ) is then formed on the stress reducing layer ( 42 ). Thus, entering of moisture is effective prevented while the stress is reduced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic electroluminescence (EL)panel having organic EL elements arranged in a matrix on a substrate,and more particularly to such an organic EL panel which can effectivelyprevent moisture from entering the organic EL elements.

2. Description of Related Art

Organic EL display panels have conventionally been known as one exampleof flat display panels. Because, unlike liquid crystal display (LCD)panels, organic EL display panels are self emissive and because organicEL display panels are bright and clear flat display panels, theirwidespread use is highly expected.

An organic EL display comprises a great number of organic EL elementsarranged in a matrix, and employs these organic EL elements as pixels ofa display. Such organic EL displays can be driven passively or actively,similar to LCDs, and, also as is the case with LCDS, active matrixdisplays are more preferable. More specifically, in active matrixdisplays, switching elements (typically two elements including aswitching element and a driving element) are provided for each pixel anddisplay of each pixel is controlled by controlling the switchingelements, whereas in passive driving, a switching element is notprovided for each pixel. Of these two types of displays, the activematrix is more preferable because much more precise displays can beachieved.

The organic EL element emits light when current flows through an organicemissive layer. In many cases, a hole transport layer and an electrontransport layer formed of an organic material are provided adjacent tothe organic emissive layer so as to assist light emission. These organiclayers are apt to degrade when exposed to moisture.

Accordingly, in organic EL displays, in order to remove moisture fromthe organic layers, a cathode formed of a metal is provided for coveringthe upper portion of the organic EL element while a space above thedisplay region (a region where pixels are provided) in which the organicEL element is disposed is made air proof and a drying agent is providedin this space.

However, there remains a demand for more reliable prevention ofdiffusion of moisture into the organic layers of the organic EL element.Although it is possible to increase the thickness of the cathode as asolution, for example, when the thickness is too large, the cathodewould be deformed and peeled due to the resulting stress.

SUMMARY OF THE INVENTION

The present invention advantageously enables reliable prevention ofmoisture diffusion.

In accordance with one aspect of the present invention, a moisture blocklayer is provided on a cathode so that intrusion of moisture into theorganic EL element, which is formed under the cathode, can be preventedmore reliably. Further, because the moisture block layer is provided viaa stress reducing layer, not directly, on the cathode, the stress of themoisture block layer and of the cathode can be suppressed anddeformation of these layers can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing a main portion of aconfiguration according to an embodiment of the present invention;

FIG. 2 is a cross sectional view showing a main portion of anotherconfiguration according to the embodiment of the present invention; and

FIG. 3 is a flowchart showing a manufacturing procedure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will be described withreference to the accompanying drawings.

FIG. 1 is a cross sectional view showing a main portion of one preferredembodiment of the present invention. On a glass substrate 10, aninsulating layer 12 formed of two layers, an SiO₂ layer and an SiNxlayer, which are accumulated in this order, is provided so as to preventimpurities from entering the glass substrate 10. On predeterminedportions of the insulating layer 12, a great number of thin filmtransistors are formed. Although in FIG. 1, a second TFT which is a thinfilm transistor for controlling current flowing from a power source lineto the organic EL element is shown, a first TFT (a switching TFT) isalso provided for each pixel for controlling accumulation of voltagessupplied from the data line into a capacitor. The second TFT is switchedon in accordance with the voltage accumulated in the capacitor forcontrolling current flowing from the power source line to the organic ELelement.

On the insulating layer 12, a semiconductor layer 14 which is formed ofpoly-Si and constitutes an active layer, is formed. Then, a gateinsulating film 16 formed of two layers, an SiO₂ layer and an SiNxlayer, which are accumulated in this order, is formed so as to cover thesemiconductor layer 14. A gate electrode 18 formed of Mo or the like isprovided via the gate insulating film 16 above the center portion of thesemiconductor layer 14. An inter-layer insulating film 20 formed of twolayers, an SiNx layer and an SiO₂ layer, which are accumulated in thatorder, is further formed so as to cover the semiconductor layer 14, thegate insulating layer 16, and the gate electrode 18. Further, towardsends on the semiconductor layer 14, a drain electrode 22 and a sourceelectrode 24 made of aluminum, for example, are formed by forming acontact hole through the inter-layer insulating film 20 and the gateinsulating film 16.

Then, a first planarization film 26 formed of an organic material suchas an acrylic resin is formed so as to cover the inter-layer insulatingfilm 20, the drain electrode 22, and the source electrode 24. On thefirst planarization film 26, a transparent electrode 30 formed of amaterial such as ITO is formed so as to serve as an anode of the organicEL element for each pixel.

A part of the transparent electrode 30 extends down on the sourceelectrode 24, where the transparent electrode 30 is formed along theinner surface of a contact hole through which the upper end of thesource electrode 24 is exposed. In this manner, the source electrode 24is directly connected with the transparent electrode 30.

Further, portions of the transparent electrode 30 formed in theperipheral region of the pixel area, which corresponds to an area otherthan the emissive region, are covered with a second planarization film32 formed of an organic material which is similar to that used for thefirst planarization film 26.

A hole transport layer 34 is then formed over the entire surface of thesecond planarization film 32 and the transparent electrode 30. With sucha configuration, because the second planarization film 32 has an openingin the emissive region, the hole transport layer 34 is in direct contactwith the transparent electrode 30, which serves as an anode, in theemissive region. On the hole transport layer 34, an emissive layer 36and an electron transport layer 38 which are slightly larger than theemissive region are sequentially accumulated in this order for eachpixel. On these layers, a cathode 40 made of aluminum or the like isformed. It is preferable that the cathode 40 is formed of lithiumfluoride (LiF) and aluminum (Al) which are sequentially formed in thisorder in a laminate structure.

Accordingly, when the second TFT is turned on, current is supplied viathe source electrode 24 to the transparent electrode 30 of the organicEL element, and flows between the transparent electrode 30 and thecathode 40. In accordance with the current, the organic EL element emitslight.

According to the present embodiment, a stress reducing layer 42 and amoisture block layer 44 are sequentially formed in this order so as tocover the whole surface of the cathode 40. The stress reducing layer 42is formed of an organic material, particularly preferably an organicmaterial which forms the organic EL element. In this embodiment, Alq₃ isused for the stress reducing layer 42. Alq₃ is a material which emitsgreen light and is used for the emissive layer 36, and is also used forthe hole transport layer 34 for the purpose of utilizing its holetransporting ability. It is also preferable that a material which is thesame as that used for the electron transport layer 38 is used for thestress reducing layer 42. Furthermore, CuPc having electron transportingability can be used instead of Alq₃.

Further, the moisture block layer 44 which is formed on the stressreducing layer 42 is formed of an inorganic material which does notallow transmission of moisture, and is formed, in this embodiment, by alayer having a laminate structure of lithium fluoride and aluminum whichis the same as that used in the cathode 40. Further, silicon nitride(SiNx), molybdenum (Mo), or the like may also be used for the moistureblock layer 44. A UV resin, Siox, SiON or the like may further be used.

As described above, in this embodiment, the moisture block layer 44 isformed on the cathode 40 via the stress reducing layer 42. It istherefore possible to reliably prevent moisture from diffusing into theorganic layers of the organic EL element formed under the cathode 40. Inparticular, due to provision of the stress reducing layer 42, which isformed of an organic material and functions as a layer which reducesstress, a problem that the cathode 40 removes off the organic layers dueto the stress, for example, can be prevented.

Further, by forming the stress reducing layer 42 immediately afterformation of the cathode 40, the stress reducing layer 42 can serve as aprotective layer for the cathode 40. It is therefore possible to reduceadverse effects on the cathode 40 even when the moisture block layer 44is formed in a separate vacuum chamber by moving the substrate to thechamber, for example.

While materials for the stress reducing layer 42 and the moisture blocklayer 44 are not limited to those described above, because the materialsdescribed above are also used for forming the organic EL panel of thepresent embodiment, when these materials are used it is possible to formthe stress reducing layer 42 and the moisture block layer 44 usingequipments which have been already used in the previous manufacturingsteps.

Normally, a single vacuum chamber is segmented into compartments eachused for forming each organic layer of the organic EL element and thecathode 40, and a substrate is sequentially moved to each compartmentfor forming each layer by vapor deposition. In such a case, the stressreducing layer 42 and the moisture block layer 44 are formed in thecompartments which were already used for forming other layers.

Typically, a layer of silicon nitride is formed by CVD and a layer ofmolybdenum is formed by sputtering. Accordingly, when a material used ina step other than the steps for forming the organic EL element is usedfor the stress reducing layer 42 and the moisture block layer 44, theoperation for forming these layers becomes complicated.

In the above configuration, the cathode 40, the stress reducing layer42, and the moisture block layer 44 are formed to thicknesses ofapproximately 400 nm, 150 nm, and 400 nm, respectively. Further, whileeach of the stress reducing layer 42 and the moisture block layer 44 isformed as a single layer in the above example, these layers may have amulti-layer structure having similar stress reducing layer 42 andmoisture block layer 44 accumulated thereon.

Further, FIG. 1 shows an example configuration in which the end portionsof the stress reducing layer 42 and the moisture block layer 44terminate without hanging down. As shown in FIG. 2, it is preferable tocause the end portions of the stress reducing layer 42 and the moistureblock layer 44 to hang down to the surface of the first planarizationfilm 26, thereby covering the side ends of the organic layers or thelike.

While FIG. 1, which is a cross sectional view showing a main portion,shows only one pixel having one organic EL element, a plurality ofpixels having the above configuration are actually disposed in a matrix.In a peripheral portion of the display region where these pixels aredisposed in a matrix, a horizontal driving circuit and a verticaldriving circuit for driving each pixel, as well as other connectinglines are formed. To a TFT substrate on which these circuits are formed,a sealing substrate is connected only in the peripheral region so as tomake the substrates air-proof, thereby forming an EL panel.

An organic EL panel is formed according to the procedure shown in FIG.2, for example. First, a glass substrate 10 is provided (S1), and agreat number of TFTs are formed on the grass substrate (S2). Then, ananode 30, organic layers including a hole transport layer 34, anemissive layer 36, and an electron transport layer 38, and a cathode 40are formed to form an organic EL element (S3).

After the organic EL element is formed as described above, a stressreducing layer 42 is formed (S4). In the present embodiment, the stressreducing layer 42 is formed by a material which is the same as that usedin either the hole transport layer 34, the emissive layer 36, or theelectron transport layer 38. Accordingly, as a vacuum chamber forforming the stress reducing layer 42, the vacuum chamber which was usedfor forming the organic layers of the organic EL element can be used.

Then, a moisture block layer 44 is formed on the stress reducing layer42 (S5). In the present embodiment, the moisture block layer 44 isformed by a material which is the same as that used for the cathode 40.Accordingly, as a vacuum chamber for forming the moisture block layer44, the vacuum chamber which was used for forming the cathode 40 can beused.

While the preferred embodiment of the present invention has beendescribed using specific terms, such description is for illustrativepurposes only, and it is to be understood that changes and variationsmay be made without departing from the spirit or scope of the appendedclaims.

1. An organic EL panel having a plurality of organic EL elementsarranged in a matrix on a substrate, in which each organic EL elementincludes an anode, a cathode, and an organic emissive layer disposedbetween the anode and the cathode, and the cathode is formed as a commonelectrode for the plurality of organic EL elements arranged in a matrix,the organic EL panel comprising: a moisture block layer which isprovided on the common cathode for preventing moisture from entering thelayers formed under the cathode, and a stress reducing layer which isprovided between the cathode and the moisture block layer.
 2. An organicEL panel according to claim 1, wherein the cathode and the moistureblock layer are formed of the same material.
 3. An organic EL panelaccording to claim 1, wherein the stress reducing layer is formed of anorganic material.
 4. An organic EL panel according to claim 3, whereinthe stress reducing layer is formed of a material which is used forforming an organic layer constituting the organic EL element.
 5. Anorganic EL panel according to claim 4, wherein the stress reducing layeris formed of an organic material including Alq₃ or CuPc. 6-7. (canceled)8. An organic EL panel according to claim 1, wherein the stress reducinglayer covers side ends of the organic emissive layer.
 9. An organic ELpanel according to claim 1, wherein the moisture block covers side endsof the organic emissive layer.
 10. An organic EL panel according toclaim 1, wherein the stress reducing layer covers side ends of theorganic emissive layer.